Blue book - make 100% sure the seller's name is on the book. Not a dead relative 😳 or anyone else.
If 5yrs or less, take it to a dealer (or trusted wrench) to check it.
If one owner, observe their character.
Don't fret over minor cosmetic issues, they're easily and cheaply repaired. Confirm any/all recall/warranty work done.
Considering all the above, private sale preferred over dealer.